bitbake: utils.py: Check for duplicate dependency entries

bitbake: utils.py: Check for duplicate dependency entries

explode_dep_versions is not able to have duplicate entries.  Previously
duplicate entries ended up with the last item being the one returned to
the caller.

We now detect a collision.  We do allow an empty item to have a comparison
added to it, or a duplicate with the same comparison without error.

When a collision is detected a ValueError exception is thrown.

Allowed:
   foo foo (= 1.12) foo

Invalid:
   foo (= 1.12) foo (= 1.13)

(Bitbake rev: d40448f0483a2959e9dcaac9b6dd35839f396a6e)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>